In a ring laser gyroscope (RLG), to obtain accurate inertial rotation information, a constant pathlength has to be maintained. To do so, U.S. Pat. No. 4,320,094 teaches the use of an electronic circuit for calculating the respective power of the circulating light beams in the cavity of a RLG. The power thus calculated is then used to modulate a piezoelectrically activated movable mirror assembly for maintaining the pathlength constant. In copending application Ser. No. 034,377, filed on Apr. 6, 1987 and assigned to the same assignee of the instant invention, a polyhedron having three facets including one for a fringe photodetector and the remaining two for power monitoring photodiodes is disclosed. There, it was taught that in order to control the movement of the piezoelectric transducer for maintaining constant pathlengths for the gyroscope, the light intensities of the respective laser beams have to be measured and summed by the corresponding power monitoring photodiodes. Yet, in applications such as inertial guidance of missiles, aircraft, tanks, etc., space for the placement of a RLG is at a premium. Consequently, the size of the RLG needs to be reduced.